1. Field of the Invention
The present invention relates to an oxygen concentration detection apparatus with an adjusting device, and a method for adjusting the oxygen concentration detection apparatus.
2. Description of Background Information
In order to minimize the emissions of exhaust gas components and to improve the fuel economy of an internal combustion engine, a feedback type air/fuel ratio control system is generally used in which the oxygen concentration in the exhaust gas is detected and the air/fuel ratio of the mixture supplied to the engine is controlled to a target air/fuel ratio by a feedback control operation in accordance with a result of the detection of the oxygen concentration. As a type of oxygen concentration sensor for use in such a feedback type air/fuel ratio control system, there is an oxygen concentration sensor which generates an output signal varying generally in proportion to the change in the oxygen concentration in the gas whose oxygen concentration is to be measured. As an example, Japanese patent application laid open No. 52-72286 discloses a critical current type oxygen concentration detection device which includes a flat oxygen-ion conductive solid electrolyte member with a pair of electrodes on its main surface. The surface of one of the electrodes on the flat oxygen-ion conductive solid electrolyte member forms a part of a gas retaining chamber into which a gas whose oxygen concentration is to be measured (measuring gas) is introduced through an introduction orifice.
In this type of oxygen concentration detection device, the solid electrolyte member together with the pair of the electrodes operate as an oxygen pump unit. When a drive current is supplied between the electrodes so that the electrode located on the gas chamber's side operates as a negative electrode, oxygen in the gas filling the gas retaining chamber is ionized and migrates toward the surface of the electrode operative as a positive electrode. The oxygen ions are released through the surface of the positive electrodes in the form of the oxygen gas. The critical value of the current which can flow between the electrodes under this condition becomes substantially constant without regard to the change in the voltage of the supply current. On the other hand, it becomes proportional to the oxygen concentration in the measuring gas. Therefore, by detecting the magnitude of the critical current, the oxygen concentration in the measuring gas can be detected.
However, with the thus constructed oxygen concentration detection device, the output signal whose magnitude is proportional to the oxygen concentration is obtained only when the air/fuel ratio of mixture detected in terms of the oxygen concentration in the exhaust gas is on the lean side from the stoichiometric air/fuel ratio. Therefore, it was not possible to set a target air/fuel ratio value for the feedback air/fuel ratio control in a rich range of the air/fuel ratio.
There is another type of oxygen concentration detection device which can produce an output signal whose level is proportional to the oxygen concentraion in the exhaust gas both in the rich range and a lean range of the air/fuel ratio. As an example, there is a device which includes a pair of flat solid electrolyte members each of which are provided with a pair of electrodes. The surface of one of two electrodes provided on each solid electrolyte member respectively forms a part of the gas retaining chamber which in turn communicates with the measuring gas via an introduction orifice. The surface of the other electrode of one of the solid electrolyte members faces an atmospheric air chamber. This type of detection device is described in Japanese patent application laid open No. 59-192955.
In the case of this oxygen concentration detection device, one of the oxygen-ion conductive solid electrolyte members and its two electrodes are operative as the sensor cell unit for sensing the oxygen concentration, and the other one of the oxygen-ion conductive solid electrolyte members and its two electrodes are operative as an oxygen pump unit. With this construction, a drive current is supplied so that oxygen-ions in the oxygen pump unit move toward its electrode located on the gas retaining chamber's side when a voltage generated across the electrodes of the oxygen concentration detecting sensor cell unit is higher than a predetermined reference voltage. On the other hand, when the voltage across the electrodes of the sensor cell unit is lower than the predetermined reference voltage, the drive current is supplied so that the oxygen-ions move toward the electrode which is located on the opposite side from the gas retaining chamber. In this way, the variation of the current value becomes proportional to the oxygen concentration both in the lean range and the rich range of the air/fuel ratio.
However, in this type of oxygen concentration detection device having an output signal proportional to the oxygen concentration, its performance such as the signal output characteristic tends to vary from one product to the other through the manufacturing process of the oxygen concentration detection device which is made up of the oxygen pump unit and the sensor cell unit. In order to reduce such a variation, an increase of the production cost has been difficult to avoid. Otherwise, due to the deviation of the characteristic of the oxygen concentration detection device, the magnitude of the current supplied to the oxygen pump unit will vary from one product to the other with respect to the same value of the air/fuel ratio. Therefore, in such a case, it is difficult to detect the air/fuel ratio of the mixture accurately from the oxygen concentration in the exhaust gas.